1. Field of the Invention
The present invention relates to a secondary battery and a manufacturing method thereof, and more particularly, a secondary battery which can improve reliability of a safety device of the battery, and a manufacturing method thereof.
2. Description of the Related Art
In general, secondary batteries are rechargeable and can be made into a smaller size with high capacity, and examples of useful secondary batteries include nickel-hydrogen (Ni-MH) batteries, lithium batteries and lithium-ion batteries. Generally, a secondary battery is formed by accommodating a generation element. The generation element generally includes an electrode unit which includes a positive electrode plate, a negative electrode plate and a separator. The electrode unit and an electrolytic solution are combined to form the generation element, and the generation element is disposed in a can made of aluminum or aluminum alloy. An upper opening of the can is sealed using a cap assembly. The secondary battery sealed in the can generally includes an electrode terminal formed at an upper portion of the battery and which is insulated from the can. The electrode terminal serves as one electrode of the battery. In this case, the can of the battery, such as, the bottom surface of the battery, serves as the other electrode of the battery.
As there is demand for small, lightweight secondary batteries, instead of making the battery can of iron, such as a cold-rolled steel sheet, the battery can is made with an aluminum can made of aluminum or an aluminum alloy. Aluminum is used since aluminum is lighter than iron or other conductive metals, which helps to make lightweight batteries. Also, even when the battery can made of aluminum is used for a prolonged period, corrosion is not caused to the battery.
In a battery having a can made of aluminum, however, where an electrode terminal at an upper portion of the can is used as an electrode of the battery and a bottom surface of the battery is used as the other electrode, several problems may be caused during an initial charge/discharge stage of the battery. Charging probes that support the electrode terminal and bottom surface of the battery may be stained with aluminum used for the battery can, which may result in poor appearance of the battery and inaccurate measurement of voltage and current by the probe, causing overcharging.
The sealed secondary battery is generally electrically connected to a safety device, such as a positive temperature coefficient (PTC) element, a thermal fuse or a protecting circuit. The safety device and the sealed secondary battery are encased in a battery pack. Such a safety device, which is connected to positive and negative electrodes of the battery, prevents rupture of a battery by interrupting current flow when the voltage of the battery sharply increases due to a rise in temperature or overcharge/overdischarge of the battery, thereby preventing dangers of the battery.
A safety device of a battery is connected to positive and negative electrodes of the battery through a lead. The lead is generally made of nickel, a nickel alloy or nickel-plated stainless steel to provide a predetermined level of hardness and conductivity. However, a lead made of nickel or a nickel alloy may cause several problems when it is welded to a can made of aluminum or an aluminum alloy. In other words, the infusibility of nickel makes it difficult to perform ultrasonic welding. Additionally, the high electrical, thermal conductivity of aluminum makes it difficult to perform resistance welding due to difficulty of gaining intensive heat at the contact interface. Thus, laser welding may be employed. During laser welding, however, laser beams may be transferred to a protector, resulting in poor reliability.
To overcome the above problems, U.S. Pat. No. 5,976,729, discloses a cell having a safety device, such as a protector. A bottom plate made of nickel or a nickel alloy is laser-welded to a bottom surface of a can made of aluminum or an aluminum alloy. The lead is welded to the bottom plate by resistance welding. However, since the can is very thin, it is necessary to accurately adjust welding strength in order to prevent leakage of an electrolytic solution from a laser-welded portion. Also, since an additional welding step is necessary, the process becomes complex. Further, since laser welding requires an increased facility, the manufacturing cost increases. If a safety vent is provided at the bottom portion of the can, thermal impact may be applied to the safety vent during laser welding, lowering reliability.
Japanese Patent Publication No. hei 8-329908 discloses a battery having a nickel plate compressively adhered to the bottom of a can made of aluminum. In this case, since the nickel plate is inserted into the bottom of the can by compressive adhesion using a physical force, the bottom portion of the can made of aluminum may become weak due to its reduced thickness, resulting leakage of an electrolytic solution. To overcome the drawback, the bottom portion of the can may be thickened, which may adversely affect the overall size of the battery.